Switching 2100 watt transformer

[Rob Howe]

I have a 2100 watt transformer to power my 12 volt LED strips. I have 150' of strips at 4.4 watts per foot or a total of 660 watts. I have only hooked up 33 feet so far and find they are using about 1.47 watts per foot. I would like to control the transformer (ON/OFF) using an Insteon device rated for 1800 watts. I think this should be OK, but the folks at SmartHome said no. I think the circuit will be around 220 watts total and therefore the switch would be OK.

 

[Tony Stewart]

The problem lies in how you diode bridge the transformer with a large cap. The % ripple you choose will be inversely affecting the peak surge current when turned on.
Thus a 10% ripple voltage results in a 1000% or 10x surge current, much like an incandescent light bulb but may differ depending on your selection and if you have a series choke to reduce Ipk. It may or may not handle this.

The second problem is the inductance of the transformer may be many Henries and depending on the protection built into Insteon's MOV surge suppressor may exceed the Joules of transient energy when switched off. (1/2LI²) ( you could add a bigger MOV)

These SMD reels of LEDs are rated for automotive voltages, so the power will depend on voltage being 12.0 or 14.2V

If you want max brightness and low flicker use 14.2V with a small battery & charger and switch the DC.
If you want cheap solution use 12.0V from PC PSU's

Check specs on 12V for a ~850W+ PSU from a PC and use the 5V enable line to gnd to switch the load on the PC connector.

I would prefer you use two old 500W PSU and check specs on power avail for 12V only. Some old PSU's require a pre-load on +5V of 5% to turn on.

If you daisy chain 160' (50m or 10 reels) you will have I max track current issues.

Distribute power separately as the current from cascading more than two 5m reels may be excessive for the printed FPC tracks.

I use 16 or 18AWG wire to feed the strings in a loop to reduce current and voltage drop. You can decide how much I max & V drop on distribution you can handle.

I have used 200W transformers before that were pre-installed for 12Vac 8W lamps with a 30A bridge and 10mF successfully for LED power, but I used a electro-mechanical timer AC switch.

 

[Diver300]

A big transformer will have a big turn-on surge. You can limit that with an NTC thermistor in series with the primary. To limit as much as possible, you should rate the NTC for the actual load, not the transformer rating.

https://www.farnell.com/datasheets/1899132.pdf

These will reduce the inrush current, and then heat up to reduce the losses when the transformer is running.

 

[Colin]

How often do you want to turn the transformer on and off.

 

[Tony Stewart]

What is your transformer voltage ratio and what rectifier design parts are you planning? ( if not using my recommendation)

Here's an example of a bad implementation.

Note the 660W of LEDs will have high power losses everywhere from high excitation current of transformer and high Cap. ripple current and ESR of equivalent Zener loads.
Values are just an estimate.